What to do when a dependency can be expensive to create?

Question

I wrote a simple program to test Cloudflare's wirefilter, here is an example that works.

use wirefilter::{ExecutionContext, Scheme};

lazy_static::lazy_static! {
    static ref SCHEME: Scheme = Scheme! {
        port: Int
    };
}

#[derive(Debug)]
struct MyStruct {
    port: i32,
}

impl MyStruct {
    fn scheme() -> &'static Scheme {
        &SCHEME
    }

    fn execution_context(&self) -> Result<ExecutionContext, failure::Error> {
        let mut ctx = ExecutionContext::new(Self::scheme());
        ctx.set_field_value("port", self.port)?;

        Ok(ctx)
    }
}

fn main() -> Result<(), failure::Error> {
    let data: Vec<MyStruct> = (0..10).map(|i| MyStruct { port: i as i32 }).collect();
    let scheme = MyStruct::scheme();
    let ast = scheme.parse("port in {2 5}")?;
    let filter = ast.compile();

    for i in data
        .iter()
        .filter(|d| filter.execute(&d.execution_context().unwrap()).unwrap())
    {
        println!("{:?}", i);
    }

    Ok(())
}

this will print:

MyStruct { port: 2 }
MyStruct { port: 5 }

If I create the vector data after I validate the schema, the borrowing system will start to complain.

I would like to validate the user input "port in {2 5}" before the creation of that vector which is an expensive operation, is there any way to do it?

The second version of the code is:

use wirefilter::{ExecutionContext, Scheme};

lazy_static::lazy_static! {
    static ref SCHEME: Scheme = Scheme! {
        port: Int
    };
}

#[derive(Debug)]
struct MyStruct {
    port: i32,
}

impl MyStruct {
    fn scheme() -> &'static Scheme {
        &SCHEME
    }

    fn execution_context(&self) -> Result<ExecutionContext, failure::Error> {
        let mut ctx = ExecutionContext::new(Self::scheme());
        ctx.set_field_value("port", self.port)?;

        Ok(ctx)
    }
}

fn main() -> Result<(), failure::Error> {
    let scheme = MyStruct::scheme();
    let ast = scheme.parse("port in {2 5}")?;
    let filter = ast.compile();

    let data: Vec<MyStruct> = (0..10).map(|i| MyStruct { port: i as i32 }).collect();
    for i in data.iter().filter(|d| filter.execute(&d.execution_context().unwrap()).unwrap()) {
        println!("{:?}", i);
    }

    Ok(())
}

which will fail with this message:

error[E0597]: `data` does not live long enough
  --> src/main.rs:33:14
   |
33 |     for i in data.iter().filter(|d| filter.execute(&d.execution_context().unwrap()).unwrap()) {
   |              ^^^^ borrowed value does not live long enough
...
38 | }
   | -
   | |
   | `data` dropped here while still borrowed
   | borrow might be used here, when `filter` is dropped and runs the destructor for type `wirefilter::filter::Filter<'_>`
   |
   = note: values in a scope are dropped in the opposite order they are defined

It seems that I can parse the query before data is created, but I cannot compile it.

score 1 · Accepted Answer · answered Apr 28 '19 at 09:28

1

You can separate the declaration of data from its assignment:

let data: Vec<MyStruct>;
let scheme = MyStruct::scheme();
let ast = scheme.parse("port in {2 5}")?;
let filter = ast.compile();
data = (0..10).map(|i| MyStruct { port: i as i32 }).collect();

The lifetime of data is the same as in the first version of your code, while the assignment happens at the same time as in the second version.

answered Apr 28 '19 at 09:28

Sven Marnach

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Cool, thanks for the answer! Do you know how to make the same principle when you are working with a stream of data? I tried the same approach witha an interator and it does not seem to work too. ``` fn main() -> Result<(), failure::Error> { let data = generate_data(); ... let filtered = data.filter(|my_struct| filter.execute(&my_struct.execution_context().unwrap()).unwrap()); Ok(()) } fn generate_data() -> impl Iterator { (0..100).map(|port| MyStruct { port }) } ``` – Jayson Reis Apr 28 '19 at 10:50
@JaysonReis The code is not readable in a comment, and "it does not seem to work" is not a useful problem description. Please ask a new question, possibly referencing this question if appropriate. – Sven Marnach Apr 28 '19 at 11:14
You are right, I will re-write the code and post another question. – Jayson Reis Apr 28 '19 at 14:56

What to do when a dependency can be expensive to create?

1 Answers1